Small stepping motors, which may be installed in cell phones or PDAs, etc. to drive their camera lenses, must be provided with reduction gears and cams to convert high speed rotation into linear motion. Furthermore, in conventional small stepping motors, when rotated or reversely rotated, backlash occurs, thus resulting in error. Therefore, such small stepping motors have been limitedly used. In addition, the small stepping motor is problematic in that high electric current is required and excessive heat is generated.
Generally, in methods of driving linear motors using piezoelectric or electrostrictive substrates, there are a driving method of using a traveling wave generated by a flexural wave, and a driving method which uses a standing wave and in which a linear motor is provided with both a longitudinal vibration actuator and a transverse vibration actuator so that a movable unit is operated by repeated vertical and horizontal vibration. Standing wave type linear motors are provided with vibrators having different operating modes and use multiple vibrations generated by them. Such a standing wave type linear motor includes a piezoelectric/electrostrictive actuator which vibrates vertically and horizontally, and a contact part which transmits mechanical displacement to a movable body which is moving. Longitudinal vibration of a piezoelectric vibrator is transmitted to the contact part at which the movable unit is coupled to the piezoelectric vibrator. The movable body is operated by friction at a junction between it and the movable unit. In the meantime, several other vibration transmitting methods have been proposed, but, because maintaining constant vibration amplitude is difficult due to wear resulting from repeated motion over a long period of time, it is very hard to put into practical use.
First, before preferred embodiments of the present invention are explained in detail, a piezoelectric effect and vibration theory which are basic theories applied to the present invention will be described herein below for comprehension of the present invention.
Piezoelectric effect means that an electric charge is generated in a crystalline body when the crystalline body receives pressure, or, conversely, when an electric field is applied to the crystalline body, the crystalline body is mechanically displaced. A piezoelectric substrate 10 having such piezoelectric effect is characterized in that mechanical displacement is induced according to the polarization direction and the direction of the electric field.
FIG. 1 shows mechanical displacement of the piezoelectric substrate 10 according to the polarization direction and the direction of the electric field.
FIG. 1(a) shows displacement of the piezoelectric substrate 10 when an electric field is applied to the piezoelectric substrate 10 polarized in a predetermined direction. When the polarization direction of the piezoelectric substrate 10 is the same as the direction of the electric field, the piezoelectric substrate 10 is expanded in a direction designated by the reference character z and is constricted by Poisson's ratio in a direction designated by the reference character x. When the polarization direction of the piezoelectric substrate 10 is opposite to the direction of the electric field, the piezoelectric substrate 10 is constricted in a direction z and is expanded in a direction x.
FIG. 1(b) illustrates displacement of the piezoelectric substrate 10 attached to an elastic body 20. In this case, the piezoelectric substrate 10 is displaced in the same manner as that described for the case of FIG. 1(a), and bending displacement of the elastic body 20 attached to the piezoelectric substrate 10 is induced by the expansion and constriction of the piezoelectric substrate 10.
The dotted line of FIG. 1(b) denotes the shape of the elastic body 20 bent when the piezoelectric substrate 10 is expanded in a direction z. Such bending displacement of the elastic body 20 is achieved by the expansion of the piezoelectric substrate 10 while a fixed edge 25 of the elastic body 20 is held at a predetermined position.
FIG. 1(c) illustrates the elastic body 20 bent in a direction z by the expansion of the piezoelectric substrate 10 in a direction x. When the direction of the electric field is instantaneously changed, the displacement state of the piezoelectric substrate 10, which was in the state of FIG. 1(b), is quickly changed. As a result, the elastic body 20 is quickly bent in a direction z by instantaneous acceleration and expansion of the piezoelectric substrate 10 in the direction x.
Although the bending displacement of the piezoelectric substrate, when an electric field is applied, has been described, even if an electrostrictive substrate is used in place of the piezoelectric substrate, the same bending displacement as that of the case of the piezoelectric substrate is induced. The electrostriction means that an electrostrictive body is mechanically displaced when an electric field is applied to the electrostrictive body. Even if the piezoelectric substrate of FIG. 1 is replaced with the electrostrictive substrate, the same bending displacement is induced.
Therefore, in the present invention, a linear motor, which induces bending displacement using the piezoelectric or electrostrictive substrate and converts the bending displacement into linear displacement, will be described herein.